It is known that reduced coenzyme Q10 can be obtained, for example, by such a method known in the art as synthesis, fermentation or extraction from a natural product, followed by concentration of the reduced coenzyme Q10 fraction in the eluent by chromatography (Patent Document 1). It is described in the above-cited patent document that, in this case, oxidized coenzyme Q10 contained in the reduced coenzyme Q10 may be reduced with an ordinary reducing agent such as sodium borohydride or sodium dithionite (sodium hyposulfite), followed by concentration by chromatography and that reduced coenzyme Q10 can also be obtained by the method comprising reacting the above-mentioned reducing agent with the existing high-purity coenzyme Q10.
However, it is not always possible to recover the thus-obtained reduced coenzyme Q10 in a highly pure condition; rather, it is often obtained in the form of, for example, low-purity crystals or an oily substance or semisolid containing oxidized coenzyme Q10 and other impurities.
The present inventors have made intensive investigations and established several methods for obtaining high-quality reduced coenzyme Q10, and have applied for patent (e.g. Patent Documents 2 to 4).
However, reduced coenzyme Q10 is readily oxidized to oxidized coenzyme Q10 by molecular oxygen and even when high-quality reduced coenzyme Q10 is produced by such methods as disclosed in the above-cited patent applications, it is still an important problem how to stabilize reduced coenzyme Q10 in processing reduced coenzyme Q10 into foods, functional nutritive foods, specific health foods, nutritional supplements, nutrients, animal drugs, drinks, feeds, cosmetics, medicines, remedies, preventive drugs and the like or raw materials or compositions therefor and/or storing them after processing. It is very difficult to completely eliminate or shut out oxygen in the above-mentioned processing and storing, and residual oxygen or newcomer oxygen, in particular, exerts great adverse influences during warming in the step of processing and/or during a long period of storage. The above-mentioned oxidation is confronted with a quality problem, namely the formation of oxidized coenzyme Q10 as a byproduct.
Thus, it is a very important problem to stabilize (protect against oxidation) reduced coenzyme Q10. Since, however, reduced coenzyme Q10 is not yet available on the market, few investigations have been made concerning the method for stably maintaining reduced coenzyme Q10 or the composition therefor. There are only a few reports describing, for example, a composition comprising a reducing agent caused to coexist and a method for preparing the same (Patent Document 5) and a stabilized composition comprising reduced coenzyme Q10 and an oil and fat and/or a polyol (Patent Document 6).
In the above-cited Patent Document 5, the following are disclosed:
1) A composition comprising reduced coenzyme Q10, an effective amount of a reducing agent for inhibiting the reduced coenzyme Q10 from being oxidized into oxidized coenzyme Q10, and an effective amount of a surfactant or a vegetable oil or a mixture of these for dissolving the reduced coenzyme Q10) and the reducing agent, if necessary together with a solvent;
2) A composition for oral administration as prepared by making the above composition into gelatin capsules or tablets; and, further,
3) A method for preparing the above composition containing reduced coenzyme Q10 produced in situ using oxidized coenzyme Q10 and a reducing agent.
However, whereas it describes the preparation method or composition, the above Patent Document 5 does not refer to the quality of reduced coenzyme Q10 contained in the composition (for example, the weight ratio of reduced coenzyme Q10 to oxidized coenzyme Q10) at all. Further, while it describes that reduced coenzyme Q10 is stabilized, it contains no detailed description of the stabilizing effects or examples; hence what the extent of stabilization can be expected is not clear. Further, the above-mentioned composition and the method for preparing the same are very complicated and troublesome since the composition is given a plurality of roles (namely, the first role as a field of reaction for reducing oxidized coenzyme Q10 to reduced coenzyme Q10 and the second role in stably maintaining the reduced coenzyme Q10). Furthermore, the reaction mixture as such is used in the above-mentioned composition and the method for preparing the same and, therefore, the composition can hardly be said to be safe. Although the surfactants specifically disclosed in the above Patent Document 5 are Span 80 and Tween 80, the use of Tween 80 and like polyoxyethylenesorbitan fatty acid esters is legally limited in some countries and therefore these substances cannot be used in some cases.
On the other hand, Patent Document 6 describes that reduced coenzyme Q10 is stabilized in the presence of an oil and fat or a polyol and that a polyglycerol fatty acid ester, more specifically a polyglycerol fatty acid ester with a polymerization degree being 2, is a surfactant which will not hinder the stabilizing effect of such oil and fat or polyol.    Patent Document 1: Japanese Kokai Publication Hei-10-109933    Patent Document 2: WO03/06408    Patent Document 3: WO03/06409    Patent Document 4: WO03/32967    Patent Document 5: WO01/52822    Patent Document 6: WO03/062182